With the recent developments of increase of sensitivity and miniaturization of silver halide color negative films, the demand for color negative photographic materials having further increased sensitivity and excellent image quality has been increasing. In addition, the demand for photographic silver halide emulsions to meet exacting requirements of photographic performances, such as high sensitivity, high contrast, excellent graininess, and sharpness has been increasing accordingly.
In order to meet these requirements, there have been proposed techniques of using tabular grains, obtaining improvements in sensitivity inclusive of color sensitization efficiency by sensitizing dyes, relationship between sensitivity and granularity (i.e., ratio of sensitivity to granularity), sharpness, and covering power, as disclosed in U.S. Pat. Nos. 4,434,226, 4,414,310, 4,433,048, 4,414,306, and 4,459,353.
Further, JP-A-58-113930, JP-A-58-113934 and JP-A-59-119350 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") disclose multilayer color photographic materials having improved sensitivity, graininess, sharpness, and dot reproducibility, in which tabular grains having an aspect ratio of 8:1 or more are used in a high-sensitive silver halide emulsion layer. According to these disclosures, use of tabular grains in a blue-sensitive emulsion layer brings about an improvement in sharpness owing to the low scattering properties of the grain. Their use in a green-or red-sensitive emulsion layer brings about an improvement in graininess.
JP-A-61-77847 proposes a multilayer color photographic material having improved sharpness and improved color reproducibility, in which tabular grains having an aspect ratio of 5:1 or more are used in a high-sensitive silver halide emulsion layer and a mono-dispersed silver halide emulsion is used in a low-sensitive emulsion layer.
In addition, Research Disclosure 25330 discloses a technique of controlling thickness of tabular grains. In this technique, reflection of light to which the silver halide light-sensitive layer provided over the layer containing the tabular grains is sensitive by the tabular grains is made greater so as to increase sensitivity of the light-sensitive layer, or the reflection is minimized so as not to impair sharpness of the upper layer.
As stated above, tabular grains having a high aspect ratio have various advantages to be made use of. Nevertheless, when applied to a so-called successive layer structure widespread in color photographic materials, such as for example, in which a support has provided thereon a red-sensitive layer, a green-sensitive layer, and a blue-sensitive layer each having a different sensitivity, in this order, it has been experimentally proved that use of tabular grains of high aspect ratio in light-sensitive layers, except the farthest from the support, particularly in a green- or red-sensitive layer, results in deteriorated sharpness in the low frequency side.
It has been suggested that this drawback can be overcome by using tabular grains whose uniformity is increased by specifying a relationship between a distance between twinning planes and a thickness of grains, as disclosed in Japanese Patent Application No. 311130/86 (corresponding to JP-A-63-163451). Although an emulsion comprising such tabular grains produces remarkable effects on improving sharpness, it has a tendency to cause fog on chemical ripening. This effect of inhibiting sufficient chemical ripening and, as a result, a so-called low intensity reciprocity law failure, drastically affects the commercial utility of these photographic materials.